This invention describes a process for preparing isotactic poly-alpha olefin of melt flow equal to or less than 8 dg/min comprising the steps of: A. providing a suitable catalyst system capable of producing isotactic poly-alpha olefin comprising a metallocene-type catalyst, an alumoxane as cocatalyst or catalyst activator, and optionally a catalyst support; B. ripening the suitable catalyst system of step A by contacting with an alkyl aluminum for a period of time of from 6 hours to 14 days ; C. polymerising a suitable monomer using the ripened catalyst system of step B ; and recovering the desired isotactic polyolefin.